I also should mention I think the arenas were made bigger for (here comes people complaining) the DLC modes. Things like Versus Mode, Ustanak Mode, Siege Mode, and the likes. Pretty sure (though not certain) those will use the same stages, so I'm guessing they were designed with them all in mind.
Its a shame mercenaries, a big part of recent mainline RE's, will suffer because of this. Looking at that RE5, you can see how almost immediately you are being attacked by 3 or 4 enemies at once.
In the RE6 video, you actually have to look for the enemies yourself most of the time. Bigger maps and what appears to be less enemies, i don't know...